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Defence in depth
A defence-in-depth concept is generally adopted in design and operation of nuclear power stations to ensure nuclear safety. It comprises several levels of multiple protection, including multiple reactor protection systems, engineered safety features, emergency operating procedures, on-site and off-site emergency plans, barriers to prevent release of radioactive materials, etc. The concept of defence-in-depth includes accident prevention and mitigation aspects as summarized below :
     
  Design and operation aspects include :
    1. adoption of conservative design;
    2. quality control;
    3. stringent operation procedures;
    4. surveillance to detect degradation or failure; and
    5. establishment of a safety culture.
       
  Reactor protection system
    1. Multiple reactor protection systems operating on different principles are provided so as to promptly shutdown the reactor in case of abnormality.
    2. Each reactor protection system also has built-in redundancy satisfying the single failure criterion (i.e. the system must be capable of performing its task in the presence of any single failure).
       
  Barriers
      Products of nuclear fission are trapped within the fuel pellets and claddings. There are basically three barriers (i.e. fuel cladding, primary circuit and containment) to prevent the release of these fission products to the environment.
       
  Engineered safety features
      These systems are installed to protect the plant in case of an accident so as to provide sufficient cooling to maintain the reactor at a safe stable shutdown condition. Major engineered safety features include :
       
      1. safety injection system - used to inject borated water into the reactor
      2. containment spray - used to cooldown and depressurise the containment
      3. auxiliary feedwater system - used to remove the residual heat from the reactor via the steam generator
       
      Each engineered safety feature has a duplicated system satisfying the single failure criterion and its power supply is also backed up by emergency diesel generator.
       
  Sand filter system
      In the event of a serious nuclear accident, radiological materials may be released to the containment from the reactor system. If necessary, the sand filter system can regulate the release of radioactive materials from the containment to the environment to avoid containment failure due to overpressurisation. In this circumstance, the sand filter system would filter/minimize the radioactivity released to the environment.
       
  Emergency planning
      GNPS and LNPS have in place combined comprehensive on-site and off-site emergency plans to deal with different emergencies arising from the unlikely occurrence of accidents at various levels and to activate the corresponding and necessary emergency response actions.

The above information is provided by EMSD
EMSD

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Last revision date: <19 Dec 2012>